1. Technical Field
The present teaching relates to method and system for batteries. More specifically, the present teaching relates to method and system for solar batteries and systems incorporating the same.
2. Discussion of Technical Background
In today's world, the level of energy consumption is ever increasing yet the sources of energy are limited. More and more often, solar energy is becoming an increasingly popular source of energy. To utilize solar energy, it is commonly known that a solar panel is used to acquire light energy and then transform the acquired light energy into power. Such generated power is often used to charge a battery which can then be used to provide power.
Each solar panel has an operating point at which maximum output power is produced. For a given amount of light energy, a solar panel has non-linear output electrical characteristics. As current drawn from the panel is increased, the panel output voltage falls monotonically. This is shown in FIG. 1, where the x-axis represents current drawn from a solar panel and the y-axis represents the voltage output from the solar panel. As can be seen, after the current drawn from the solar panel exceeds a maximum operating point, the output voltage from the solar panel drops drastically. For example, at 50% illumination level, when the current drawn exceeds 50 mA, the output voltage drops rapidly from 15 v to close to zero. Similarly, at 100% illumination level, when the current drawn exceeds 100 mA, the output voltage drops rapidly from 15 v to close to zero.
The power produced by a solar panel can be computed by multiplying its output voltage with its output current. Due to the above discussed behavior of a solar panel, it is known that, with respect to output power, the behavior of a solar panel can be approximated by a parabolic curve, as shown in FIG. 2. This plot shows that the output power is maximal when the drawn current reaches a certain point and after that, the output power drops significantly. For example, at 50% illumination level, as the current drawn increases, the output power increases. When the current drawn exceeds 50 mA, the output power drops rapidly from almost 700 mW to close to zero when the drawn current approaches 70 mA. Similarly, at 100% illumination level, as the current drawn increases, the output power increases accordingly. When the current drawn exceeds 100 mA, the output power drops rapidly from 1400 mW to close to zero when the drawn current approaches 130 mA.
If the current load on a solar panel is controlled so that the solar panel operates with a load corresponding to the maxima of the panel's output power characteristic for a given illumination level, the solar panel can produce the most power possible for the given illumination level. This is known as operating at a solar panel's maximum power point.
Various maximum power point tracking (MPPT) control schemes exist to operate a solar panel at its maximum efficiency. Many of these schemes directly monitor the output power of the panel, and continuously adjust the load current so that the maximum output power is maintained. These systems sweep the load on the solar panel while monitoring the output voltage and current of the panel. The monitored output voltage and current terms are multiplied throughout the sweep to determine the actual panel output power. When needed, the system will adjust the load so that the system can operate at the maximum achieved output power level.
Some prior art schemes further attempt to increase panel efficiency by continuously modifying the panel load while directly monitoring output power, and continuously adjusting the panel operating point to maintain maximum output power. These types of systems are usually complex and generally require a microprocessor for control. Thus, they are also expensive.
As discussed herein, when the solar panel output power is compared with solar panel output current, a parabolic characteristic is observed (FIG. 2). The parabolic curves with respect to different illumination levels are shifted and this is illustrated in FIG. 2. However, when solar panel output power is compared to solar panel output voltage, although a parabolic characteristic is also observed, the maxima of the power characteristics are relatively independent of illumination intensity. This is illustrated in FIG. 3. As seen there, no matter what the underlying illumination levels are, the peak performance point for both parabolic curves (corresponding to illumination levels 50% and 100%) remains the same (close to 15 v). Specific solar panels have a known relationship between output power capability and output voltage, and the maximum power voltage (VMP) is generally a specified parameter for commercially available solar panels.
High-performance solar powered battery chargers are designed to maximize the efficiency of power transfer from a solar panel to a battery. Such battery chargers are almost exclusively built using some type of switching DC/DC converter, as the power transfer efficiency of a DC/DC converter far exceeds that of a linear converter. A switching DC/DC converter can be viewed as a power transfer device, transferring power from an input supply to a load. When a DC/DC converter is powered by a solar cell, as the power requirements of the DC/DC converter increases, the power output from the solar panel must similarly increase.
When a solar panel provides increased power and its output current rises, the panel output voltage falls. The specific voltage and current for a given power output characteristic follows that of the specific panel, which is similar in shape as those characteristics shown in FIGS. 1-3. The maximum power available is achieved by operating at the maximum power point of the panel, corresponding to panel operation at the maximum power output voltage (VMP). If the power required by the DC/DC converter exceeds the power available from the solar panel, the panel voltage will fall lower than VMP, at which point the output power begins to fall. As further reductions in panel output voltage cause further reductions in output power, the panel output voltage quickly collapses.
A switching battery charger is one where the battery charging current is generated by a DC/DC converter. Switching battery charger control techniques exist in the public domain that aim at improving solar panel operational efficiency. These techniques take advantage of the characteristic collapse in panel voltage when the load on the solar panel exceeds the available output power. These techniques commonly employ a hysteretic under-voltage lockout that disables the DC/DC converter when the solar panel collapses below a reference voltage (VMP(REF)) and then re-enables the DC/DC converter once the panel voltage recovers to reach some voltage above that reference. One example of such a circuit is shown in FIG. 4(a).
The battery charging circuit shown in FIG. 4(a) comprises a switching battery charger 440, that provides an output current based on a /SHDN input. When the /SHDN input is logic high, the battery charger operates normally, and provides a charging current (Iout) to battery 450. When the /SHDN input is logic low, the battery charger is disabled, and the output current Iout=0 A.
The /SHDN input is driven by a hysteretic comparator 420 that monitors the input voltage from an input power source to the switching charger 440. The input voltage corresponds to the solar panel output voltage when the power is supplied by a solar panel. The comparator 420 compares the input voltage with a comparator reference voltage 430. To approach maximum power transfer, the comparator reference 430 needs to be set close to the panel maximum power voltage (Vmp). When the solar panel voltage rises such that the positive input of the comparator exceeds the comparator voltage reference 430 (Vmp[ref]) by the comparator hysteresis voltage (Vhyst), the output of the comparator will be driven high, and the charger will be enabled.
If the input power required by the battery charger 440 is greater than the output power available from the solar panel, the panel will be loaded beyond it's maximum power level, and the voltage on the panel will collapse. Once the panel voltage falls below Vmp[ref]-Vhyst, the comparator output will be driven logic low, and the battery charger will be disabled. This subsequently removes the loads on the solar panel, which allows the panel voltage to rise until Vmp[ref]+Vhyst is reached, wherein charger enable/disable cycle repeats. This control technique used in this prior art solution is an non-linear approach.
By setting the hysteretic thresholds within the bounds of normal converter operation, the output power delivered approaches the peak power available from the solar panel through pulse-width-modulation of the DC/DC converter input current. The greatest efficiency is possible by setting the hysteretic under-voltage thresholds as close to VMP as is practical. The solar panel output current is continuous due to integration by the converter input capacitance, but the panel operates at output voltages that are both below and above the maximum power voltage. This is shown in FIG. 4(b). Because of that, the efficiency of such approaches suffers.